Session W30: Focus Session: High Pressure IV: Dynamics of Shock Induced Phase Transitions

Physical Transformation of Matter at High Energy Density and Multi-Phase Equation of State

The experimental investigation of equations of state, adiabatic compressibility and dc, ac electrical conductivity of hot dense matter shocked and quasiisentropically compressed by reverberating shock waves up to megabar pressure range are presented. HE-driven generators of intense shock waves were used for generation of dense strongly non-ideal plasma with intense interparticle interaction and Fermi-Boltzmann types of statistics. The thermodynamic measurements demonstrate density increase at megabar pressure just in the density range where the electrical measurements have shown five orders of magnitude electrical conductivity increase due to pressure ionization. These thermodynamic experimental data in combination with the electrical conductivity measurements were interpreted as the experimental evidence of a phase transition in strongly non-ideal plasma. The existence of this new phase transition is supported by the ab initio Quantum Monte-Carlo, Density Functional Theory, and Molecular Dynamic computer simulations. Pressure ``dielectrization'' in shock compressed Li, Na, Ca was also detected. The semi-empirical multi-phase equation of state of materials in a broad region of phase diagram is constructed. The results of 2 D and 3 D computer simulation of high energy density phenomena based on this semi-empirical EOS are presented.   

Ultrafast observation of shock compression from greater than 10 GPa precompression

For decades, many compression experiments have applied either static compression in a diamond anvil cell (DAC) or dynamic compression using shock waves. Although such experiments provide a wide range of material data, information off the Hugoniot or room temperature isotherm requires more specialized techniques. Further, although ultrafast laser methods have recently been applied to acoustics in the DAC and shock waves at ambient pressure, shock waves from precompressed states have not been observed with ultrafast time resolution. Shock compression of a precompressed material enables two useful experimental strategies. First, the initial state of the material may be placed, via precompression, in the proximity of a phase transition before shock compression, enabling the observation of phase transition dynamics as the material is shock compressed through the phase transition boundary. Second, since low-density materials heat more substantially than high-density materials upon shock compression, an initial pressure may be used to modulate the degree of shock heating. Here we report the application of ultrafast shock wave methods to materials which have been precompressed in a DAC, providing material information off the standard Hugoniot.   

Atomic transformation pathways from THz radiation generated by shock-induced phase transformations

We have recently made the first experimental observation of THz frequency radiation emitted from elastically strained piezoelectric GaN and utilized this new phenomenon as an ultrafast diagnostic. Using molecular dynamics simulations coupled to Maxwell's equations, we find that similarly detectable THz frequency radiation can be emitted when a wurtzite structure crystal transforms to a rocksalt structure under shock compression on picosecond timescales. We show that information about the atomic-scale transformation pathway is contained in the sign of the emitted THz electric field and information about the kinetics is contained in the time-dependence. This new phenomenon provides an avenue to experimental measurement of microscopic transformation pathways in crystals on the shortest timescales.   

Properties of Shocked Polymers: Mbar experiments on Z and multi-scale simulations

Significant progress has been made over the last few years in understanding properties of matter subject to strong shocks and other extreme conditions. High-accuracy multi-Mbar experiments and first-principles theoretical studies together provide detailed insights into the physics and chemistry of high energy-density matter. While comprehensive advances have been made for pure elements like deuterium, helium, and carbon, progress has been slower for equally important, albeit more challenging, materials like molecular crystals, polymers, and foams. Hydrocarbon based polymer foams are common materials and in particular they are used in designing shock- and inertial confinement fusion experiments. Depending on their initial density, foams shock to relatively higher pressure and temperature compared to shocked dense polymers/plastics. As foams and polymers are shocked, they exhibit both structural and chemical transitions. We will present experimental and theoretical results for shocked polymers in the Mbar regime. By shock impact of magnetically launched flyer plates on poly(4-methyl-1-pentene) foams, we create multi-Mbar pressures in a dense plasma mixture of hydrogen, carbon, at temperatures of several eV. Concurrently with executing experiments, we analyze the system by multi-scale simulations, from density functional theory to continuum magneto-hydrodynamics simulations. In particular, density functional theory (DFT) molecular dynamics (MD) and classical MD simulations of the principal shock Hugoniot will be presented in detail for two hydrocarbon polymers: polyethylene (PE) and poly(4-methyl-1-pentene) (PMP).   

Shock Induced Phase Transitions in Polymeric Nitrogen

The reported density functional molecular dynamics simulations are of a shock travelling through $\sim $4,000 atoms arranged in the equilibrium cg-N configuration equilibrated at T = 250K, P = 1 atm. Atoms within a small segment of the material given an extra velocity consistent with various desired flyer plate impact velocity. The resulting atomic trajectories show a number of complex behaviors including a phase transition to a previously unseen phase, spontaneous defect formation, and chemical reactions. The stability of the shock and the unusual properties of the above phenomena will be discussed.   

Critical velocities for deflagration and detonation in a REBO high explosive

The effects of circular voids on the shock sensitivity of a two-dimensional model high explosive crystal are considered. We simulate a piston impact using molecular dynamics and a Reactive Empirical Bond Order (REBO) model potential for a sub-micron, sub-ns exothermic reaction in a diatomic molecular solid. The probability of initiating chemical reactions is found to rise more suddenly with the piston velocity for larger voids that collapse more deterministically than smaller voids. A void of only 10 nm radius reduces the minimum initiating velocity by a factor of 4. The shock-to-detonation transition at larger velocities is also studied using a micron-long sample containing a single void (and its periodic images). The reaction yield during the shock traversal increases rapidly with velocity, then becomes a prompt, reliable detonation. A void of radius 2.5 nm reduces the critical velocity by 10\% from the perfect crystal. A Pop plot of the time-to-detonation at higher velocities shows a characteristic pressure dependence.   

Solid-solid phase transformation in shocked Cs and Ce using molecular dynamics

Cesium (Cs) and cerium (Ce) both undergo a significant ($>$10\%) volume collapse associated with an isomorphic fcc-fcc phase transformation when subject to compressive loading. Molecular dynamics (MD) simulations provide significant insight into the atomic mechanisms of plasticity and phase transformation. We report here the results of MD simulations of Cs and Ce samples subjected to shocks with pressures ranging from 0.5 to 30 GPa. An Embedded Atom Method (EAM) potential is used to model the interaction between atoms in Ce, and an EAM-like two-band second moment model potential by Ackland and Reed is used for Cs. A split wave structure is observed, with up to 3 waves: an elastic precursor followed by two plastic waves. The latter plastic wave causes the expected fcc-fcc phase transformation. An analysis of crystallography and orientation is conducted, which shows that reorientation of the lattice depends upon the original orientation of the sample.   

Characteristic curves of shockless compression from atomistic molecular dynamics

Ramp loading experiments, such as those on Sandia's Z-machine, are increasingly used to produce shockless (quasi-isentropic) compression, enabling high-pressure off-Hugoniot states to be studied. One approach used to interpret data from such experiments is the method of characteristics. For arbitrary loading trajectories characteristic curves can be used to predict the steepening of the propagating ramp wave into a shock. Here, we present a method to extract characteristic curves from molecular dynamics simulations used to model a ramp loading response. This requires calculating the adiabatic sound speed within the material as a function of position and time. This was accomplished using extensions of the Hardy method for converting the instantaneous atom properties into continuum mechanics field variables. The method has been implemented in LAMMPS using the ATC package, and has been demonstrated using Lennard-Jones argon and EAM aluminum.   

Strain Rate Effects in Shock and Quasi-Isentropic Compression of Solids

We report on large-scale non-equilibrium molecular dynamics (NEMD) simulations of shock and quasi-isentropic compression (QIC) in defective copper crystals. The atomic interactions were modeled employing a well-tested embedded-atom method (EAM) potential for Cu. We examined the deformation mechanisms and material strength for strain rates in the range of $10^9$ - 10$^{12}$ s$^{-1}$, a regime relevant to the validation of material strength models. For samples with a relatively low density of pre-existing defects, the strain rate dependence of the flow stress follows a power law with an exponent of 0.40. On the other hand, for samples with a higher density of pre-existing defects, the flow stress exhibits a narrow linear regime at strain rates above $10^9$ s$^{-1}$ and then bends over at higher strain rates in a manner reminiscent of shear thinning in fluids. Both the NEMD shocks and QIC show behavior similar to sheared fluids, which can be described by the Ree-Eyring theory of non-Newtonian viscous flow.   

High Strain Rate Effects on the Deformation of Vanadium Nanowires

Dislocation glide and deformation twinning are the two major plastic deformation modes in b.c.c. metals and alloys. In bulk materials, dislocation motion is usually dominant at high temperatures and for low strain rates, and deformation twinning is dominant under extreme conditions of high strain rates or very low temperatures. However, at nanoscales it is not clear if the behavior of the b.c.c metal is similar to the bulk. Moreover, the corresponding atomistic mechanisms which govern the choice of deformation modes are not established. In the present study, we study the effects of strain rate on the tensile deformation response of b.c.c. Vanadium nanowires by molecular dynamics simulations using an EAM potential. We find that with the increasing strain rate, the deformation mode changes form dislocation slip to a combination of dislocation slip and deformation twinning. At sufficiently high strain rates, (112)[-1-11] deformation twins are only formed in the nanowire. In addition, the yield strength of the V nanowire increases with strain rate. Interestingly, the ductility of the nanowire also increases with the increasing strain rate, and the plastic strain at very high strain rates can be almost completely recovered (up to 40\%). Analysis of the multi-layer generalized stacking fault energies shows that this phenomenon can be understood in terms of the nucleation.   

New Strength Data on Aluminum to 160GPa

Shock compression experiments were performed on 6061-T6 aluminum up to 160 GPa to probe aluminum strength in the shocked state as it passes through the melt region ($\sim $120 -- 160 GPa). Established two and three stage gas gun launch techniques were used to achieve impact velocities of 4-10 km/s. Under symmetric impact conditions, aluminum samples were shocked to states in the solid, liquid and mixed (solid-liquid coexistence) phases and subsequently released. Velocity interferometry was used to record detailed shock wave profiles from which the shocked state is determined. Strength is determined from the quasi-elastic release portion of the wave profiles. Details of the analysis will be presented along with results showing the dependence of strength on shock stress in all three phase regions. These results will also be compared to the Steinberg strength model.   

Molecular dynamics simulation of shock wave and spallation phenomena in metal foils irradiated by femtosecond laser pulse

Femtosecond laser irradiation of metals is an effective technique to create a high-pressure frontal layer of 100-200 nm thickness. The associated ablation and spallation phenomena can be studied in the laser pump-probe experiments. We present results of a large-scale MD simulation of ablation and spallation dynamics developing in $1,2,3\mu m$ thick Al and Au foils irradiated by a femtosecond laser pulse. Atomic-scale mechanisms of laser energy deposition, transition from pressure wave to shock, reflection of the shock from the rear-side of the foil, and the nucleation of cracks in the reflected tensile wave, having a very high strain rate, were all studied. To achieve a realistic description of the complex phenomena induced by strong compression and rarefaction waves, we developed new embedded atom potentials for Al and Au based on cold pressure curves. MD simulations revealed the complex interplay between spallation and ablation processes: dynamics of spallation depends on the pressure profile formed in the ablated zone at the early stage of laser energy absorption. It is shown that the essential information such as material properties at high strain rate and spall strength can be extracted from the simulated rear-side surface velocity as a function of time.   

Efficient sticking of surface-passivated Si nanospheres via phase-transition plasticity

Large-scale atomistic simulations considering a 5 nm in radius H-passivated Si nanosphere that impacts with relatively low energies onto a H-passivated Si substrate reveal a transition between two fundamental collision modes. At impacting speeds of less than 1000 m/s particle-reflection dominates. At increased speeds the partial onset in the nanosphere of a beta-tin phase on the approach followed by a-Si phase on the recoil is an efficient dissipative route that promotes particle capture. In spite of significant deformation, the integrity of the deposited nanosphere is retained. Our result explains the efficient fabrication of nanoparticulate films by hypersonic impaction, where the nanoparticle impact velocities equal 1000-2000 m/s.